At present, in a standard heat pump system or a heat pump system with a heat recovery function, since the amounts of refrigeration medium which is needed to participate in circulation in different functional modes are different, in order to realize higher performance in different functional modes, usually a reservoir is arranged in the system to regulate the amount of the refrigeration medium needed by actual operation. For example, U.S. Pat. No. 5,551,249 discloses a heat pump system with a heat recovery function, the arrangement of which is as shown in FIG. 1, wherein the system is provided with a compressor 10, a heat recovery condenser 34, a condenser 134, an evaporator 100, a reservoir 28 and two bypass valves 52, 58 in the reservoir. The system has various working modes such as refrigeration, heating and hot water production modes, also adopts the reservoir 28 to regulate the refrigeration medium and is a typical heat pump system with a heat recovery function. However, a common technical problem exists in this kind of heat pump systems, i.e., when the heat pump systems operate in a normal refrigeration mode, the refrigeration medium also is stored in the reservoir, directly resulting that quite a few of cooling capacity is attenuated and thereby directly influencing the final refrigerating capacity in the refrigeration mode. However, it is not worth to remove the reservoir for the sake of improvement in the refrigeration mode because this will influence the working performance in the working modes such as heating mode and hot water production mode. Therefore, it is desirable to design a heat pump system which can prevent the refrigeration medium from flowing through the reservoir in the refrigeration mode but can allow the refrigeration medium to normally flow through the reservoir in other modes.